Respiratory Oscillometry and Multimodal Lung Function Assessment of Elexacaftor/Tezacaftor/Ivacaftor Response in Children and Young Adults With Cystic Fibrosis

ABSTRACT

Background

To date, there is no data on forced oscillation technique (FOT) as a tool to evaluate elexacaftor/tezacaftor/ivacaftor (ETI) modulator in young patients with cystic fibrosis (pwCF).

Objective

Our study aimed to assess the effect of a 6‐month ETI treatment on FOT parameters and compare it to the effect on spirometry and nitrogen multiple‐breath washout (N₂MBW) parameters.

Method

Forty‐nine pwCF (median age 12.39 years, 59.2% male) were assessed with spirometry, N₂MBW, and FOT before and 3 and 6 months after receiving ETI. The trends in changes of spirometric and FOT parameters were compared among pwCF of different disease severity and in the three patient groups according to their mutation group, that is, F/F, F/non‐F, and non‐F/non‐F. Panel regression was applied to define the predictors of LCI.

Results

A significant change was observed 6 months after receiving ETI in height, weight, BMI, Rs5z‐score, Xs5z‐score, AXz‐score, FEV₁z‐score, FVCz‐score, FEF_25‐75z‐score, LCI and sGrs (p < 0.01). FOT followed a similar trend in parameters' change with spirometry and N₂MBW three and 6 months after ETI among different severity groups based on LCI (p < 0.05). Genotype group was found to have a minor role in ETI efficacy; a significant improvement was found in the non‐F/non‐F group in all pulmonary tests (p < 0.01).

Conclusions

Our preliminary data suggest that oscillometry parameters showed a significant improvement after ETI treatment among all genotype groups. More data are required to assess whether and which FOT parameters could be used as surrogates of LCI or spirometry for the long‐term follow‐up of pwCF.

1. Introduction

CFTR modulator therapy has been a significant intervention for people with CF who are eligible (genotype and age) and who have access [1]. Elexacaftor/Tezacaftor/Ivacaftor (ETI) therapy was approved by the Food and Drug Administration (FDA) in late 2019 and by the European Medicines Agency (EMA) in early 2020 and people with cystic fibrosis (pwCF) who have at least one allele of ΔF508 mutation are eligible to take it [2]. Many studies indicated ETI as a highly effective therapy in cystic fibrosis (CF) [3]. Recently, ETI was approved for children over 2 years of age [4].

Pulmonary function testing is valuable for monitoring airway disease and response to interventions [5]. Spirometry remains the gold standard for monitoring respiratory function in pwCF but may be relatively insensitive to early functional abnormalities that precede or accompany structural changes such as bronchiectasis. More sensitive techniques, including nitrogen multiple breath washout (N₂MBW) for lung clearance index (LCI) and the forced oscillation technique (FOT), can detect ventilation inhomogeneity and altered airway mechanics even when spirometry is normal [6]. These abnormalities often reflect early or reversible peripheral airway involvement, such as mucus obstruction, providing complementary insight into early disease and treatment response. Respiratory oscillometry is a simple, fast, and highly reproducible method for assessing respiratory function [7]. It delivers pressure waves of various frequencies (generated by a loudspeaker or vibrating mesh) to the respiratory system through a mouthpiece superimposed on natural tidal breathing. The response of the respiratory system, known as respiratory impedance, is assessed by measuring changes in flow and pressure at the airway opening. Respiratory impedance is further divided into two components: (a) respiratory resistance (Rrs), which represents the frictional resistance to airflow in the larger airways, and (b) respiratory reactance (Xrs), which reflects the elastic properties of lung tissue (elastance) and the inertia of moving air within the airways (inertance) [8]. FOT is a non‐invasive technique that requires minimal patient cooperation and, thus, can be used over a wide age range, from early childhood to late adulthood. The method has been applied successfully in patients with asthma, chronic lung diseases, and CF to monitor respiratory function and assess the response to treatment. Nevertheless, some studies doubt the usefulness of FOT in CF and the extent of correlation with other pulmonary function tests [9]. There is a paucity of data in the literature on using FOT in pwCF treated with ETI.

Our study aimed to assess the changes in lung function with ETI therapy measured by FOT and to compare the results to the standard use of spirometry and N₂MBW parameters. As secondary aims, we sought to determine if the ETI‐related trajectories of improvement were consistent across the three functional measurement modalities (FOT, N₂MBW and spirometry) when compared among subgroups defined by their disease severity and their CFTR genotype based on the presence of the ΔF508 allele (F or non‐F), that is, F/F, F/non‐F, and non‐F/non‐F groups. As an exploratory objective, the study also aimed to investigate potential predictors of LCI to characterize further factors associated with ventilation inhomogeneity. To our knowledge, this is the first study to evaluate the oscillometry parameters change before and after ETI therapy in pwCF. We hypothesized that a simple and readily accessible method like FOT could provide added value in assessing the effectiveness of ETI treatment and monitoring the progression of lung disease among pwCF.

2. Methods

2.1. Study Design

A prospective study was carried out in the CF Unit of a tertiary University Hospital. Participants were assessed by spirometry, N₂MBW, and FOT before, three and 6 months after initiation of ETI. The study was approved by the Ethics Committee of the Aristotle University of Thessaloniki School of Medicine (decision no. 116/2024). Written informed consent was obtained from the parents/guardians or the patients if they were over 18 years old. Height and weight were measured in light clothing, and body mass index (BMI) was calculated. To ensure the validity of our baseline data, the pre‐ETI assessment was strictly defined as the measurement taken immediately prior to drug initiation. Since all patients began ETI therapy within a maximum of 2 weeks of this assessment, the two time points were considered physiologically equivalent.

2.2. Study Population

CF patients who started ETI and followed up in our CF Unit were included in the study. Eligible were children and young adults with (a) a confirmed diagnosis of CF who (b) received ETI, based on their mutations (eligible pwCF), or in the case of non‐eligible patients, after approval by the National Drug Committee, (c) had not experienced a pulmonary exacerbation in the previous 3 months, and (d) were able to successfully perform lung function tests, including spirometry, N₂MBW, and FOT. Patients experiencing pulmonary exacerbations at the time of evaluation or within the preceding 3 months were excluded to avoid confounding effects on lung function measurements. Additional exclusion criteria included discontinuation of ETI due to early respiratory worsening (“ETI flare”), hepatotoxicity, or severe skin reactions, as well as any acute illness or comorbidity that could interfere with test performance or interpretation. Of the 51 pwCF initially enrolled, two homozygous ΔF508 patients were excluded due to pulmonary exacerbations within the 3 months prior to assessment. The study cohort comprised 49 patients with CF, all identified as Caucasian (median age 12.39 years, 59.2% male). The age distribution of the sample spanned the pediatric and adult spectrum, with the majority consisting of minors: 38 patients (77.6%) were under 18 years of age and 11 patients (22.4%) were 18 years or older. All participants underwent a comprehensive baseline assessment via FOT, N₂MBW, and spirometry. Twenty‐two pwCF were treated with Lumacaftor/Ivacaftor, and one patient had Tezacaftor/Ivacaftor at the initiation of ETI.

2.3. Respiratory Function Tests

Patients included in the study underwent same‐day FOT, N₂MBW, and spirometry, at three time points: at baseline (prior to ETI initiation), and at 3 months and 6 months after ETI initiation.

2.4. FOT

FOT measurements were acquired using a Resmon Pro Full V3 device (Restech, Milan, Italy) employing a 5‐11‐19 Hz multi‐frequency signal. All data acquisition adhered strictly to established methodological guidelines [7, 10] and parameter values were interpreted using age‐appropriate reference equations [11, 12]. Shortly, the patient was breathing quietly through a face mask or mouthpiece with a bacterial filter while sitting upright with their cheeks supported and a nose clip in place. Two acceptable attempts with a coefficient of variation < 10% were collected, and the mean results were reported. The device's software automatically discharged breaths interrupted by coughing or swallowing, and those with high flow rates. The resistance at 5 Hz (Rrs5), reactance at 5 Hz (Xrs5), area of reactance (AX), deltaX (difference between reactance at 5 and 19 Hz), and the frequency dependence of resistance (R5–19) were recorded. The specific conductance of the respiratory system (sGrs) was calculated from Rrs5 and adjusted for functional residual capacity (FRC) to account for differences in lung volume at rest [13]. In FOT, Rrs reflects airflow limitation in both the central and peripheral airways. Xrs and deltaX reflect changes in lung elasticity and small airway function, with more negative values indicating worse peripheral airway involvement. sGrs reflects overall airway conductance adjusted for lung volume, with lower values indicating greater obstruction. AX reflects the overall burden of abnormal reactance and increases with worsening small airway disease and air trapping. Together, these measures indicate reduced elastic recoil and progressive involvement of the peripheral airways.

2.5. N₂‐Multiple‐Breath Washout (N₂MBW)

N₂MBW measurements were performed with a flow, volume, and molecular mass analyzer (EXHALYZER D, Eco Medics, Switzerland) according to ERS/ATS guidelines [14] and analyzed with the SPIROWARE Version 3.3.1. At least two satisfactory LCI scores were obtained for each patient. The mean tidal volume during the measurement was also recorded. Each N₂MBW trial was completed when there were two consecutive tests with < 1/40 of the initial nitrogen concentration. Normal LCI values are considered lower than eight [15, 16]. An increased LCI indicates more FRC turns required to flush nitrogen from the lungs, reflecting inhomogeneous ventilation [17]. Tests were performed at least 2 h after physiotherapy or inhaled medication to avoid any changes in FRC and LCI. FOT and N₂MBW protocols were strictly standarised to guarantee matching FRC and stable breathing mechanics. Participants maintained the same seating posture (back‐supported chair with slight head extension) and precisely controlled their tidal volume to be within 10% of the volume recorded during N₂MBW.

2.6. Spirometry

Forced expiratory volume in the first second (FEV₁), forced vital capacity (FVC), and forced expiratory flow rate at 25%–75% of the vital capacity (FEF_25–75) were measured by spirometry performed according to ERS/ATS guidelines [18] with an electronic spirometer (Vitalograph 2120, Vitalograph Ltd., Ennis, Ireland). Data were expressed as % predicted for age, sex, race, and height using Global Lung Function Initiative software data (GLI 2020, Global Lung Function Initiative Task Force, available at: http://www.lungfunction.org/) and in z‐score values. Three acceptable attempts from each patient were collected, and the greater values were reported.

2.7. Statistical Analysis

Statistical analysis was conducted using R version 4.4.1. Numerical variables were presented as mean ± standard deviation (SD), age as median, and other categorical variables as absolute numbers and percentages. Data were checked for normality before analysis. Repeated‐measures analysis of variance (ANOVA) was used to evaluate differences in variables across the three study time points. When significant, paired t‐tests were used for post hoc pairwise comparisons between time points. The degrees of freedom were determined in any case according to Mauchly's test results. As measurements were repeated within the same participants, the samples were dependent. Normality of the paired differences between time points was assessed using QQ‐plots, with IQ percentiles included to aid interpretation. A p‐value ≤ 0.05 was considered statistically significant.

A two‐way analysis of covariance (ANCOVA) was used to compare post‐ETI changes (delta values at 3 and 6 months) in continuous respiratory outcomes (FEV₁, FVC, LCI, Rrs5, Xrs5, and AX) across disease severity and CFTR mutation groups. In each analysis, the baseline value of the respective parameter was included as a covariate to account for pre‐treatment differences between groups. Model assumptions were assessed using standard diagnostic procedures and were considered acceptable. Exploratory linear regression analyses were conducted to identify potential predictors of LCI. Unadjusted models assessed crude associations, while Adjusted Models 1 and 2 included relevant covariates to account for potential confounding.

To evaluate longitudinal changes over time and account for repeated measurements within individuals, a random‐effects panel regression model was applied. This approach allowed for the estimation of overall treatment effects while accounting for within‐subject correlations. Changes in spirometry and FOT parameters at 3 and 6 months were compared according to two main categorical factors: disease severity and CFTR mutation group. Disease severity was based on baseline LCI cut‐off values, and CFTR mutation status was defined by three patient groups and mutation groups based on the presence of ΔF508 allele (i.e., F/F stands for homozygous ΔF508, F/non‐F stands for heterozygous ΔF508, and non‐F/non‐F stands for no ΔF508 copies).

3. Results

Forty‐nine pwCF were included in the study. The median age of the participants was 13.88 years, and 59.2% were male. The characteristics of the study population are shown in Table 1. Feasibility for all pulmonary function tests was high, with 100% of enrolled participants successfully completing every scheduled measurement. FOT parameters identified a higher proportion of abnormal results (57.1%) compared to N₂MBW (55%) and spirometry (24.5%).

Table 1.

Characteristics of the study cohort (N = 49).

Demographics	Total (N = 49)	On modulator therapy (N = 23)	Modulator naïve (N = 26)
Age, years	13.88 (6.02–28.94)	13.27 (6.02–26.19)	12.34 (6.61–28.94)
Weight, kg	43.46 (16.85)	41.21 (16.98)	45.46 (16.82)
BMI, kg/m2	18.73 (13.84–28.20)	18.07 (13.84–25.47)	18.82 (14.61–28.20)
Sex, male (N, %)	29 (59.2)	12 (52.2)	17 (65.4)
Mutations
F/F homozygous (N, %)	22 (44.9)	21 (91.3)	1 (3.8)
F/non‐F (N, %)	20 (40.8)	2 (8.7)	18 (69.2)
Non‐F/non‐F (N, %)	7 (14.3)	0 (0.0)	7 (27.0)
Colonization
P. aeruginosa (N, %)	15 (30.6)	6 (26.1)	9 (34.6)
S. aureus (N, %)	41 (83.7)	18 (69.2)	23 (88.5)
Spirometry
ppFEV1, %	83.16 (24.57)	81.54 (26.94)	84.59 (22.71)
FVC, z‐score	0.17 (1.71)	0.32 (1.80)	0.04 (1.66)
FEV1, z‐score	−0.39 (1.75)	−0.31 (1.97)	−0.46 (1.57)
FEF25−75, z‐score	−0.88 (1.45)	−1.07 (1.43)	−0.67 (1.42)
MBW
LCI	8.91 (2.51)	8.47 (1.95)	8.76 (2.22)
FRC, L	2.07 (0.89)	2.10 (1.02)	2.05 (0.78)
FOT
Rrs5, z‐score	1.1 (1.94)	0.31 (1.53)	1.56 (2.83)
Xrs5, z‐score	−0.72 (1.89)	0.08 (1.48)	−0.45 (1.82)
AX, z‐score	−0.44 (2.47)	−0.48 (2.39)	−0.40 (2.59)
sGrs, cmH2O‐1•s‐1	0.11 (0.04)	0.11 (0.05)	0.11 (0.04)
deltaX5, cmH2O•s•L‐1	0.2 (1.02)	−0.04 (0.63)	0.41 (1.25)

Note: Data are given as mean (SD) unless stated otherwise. Age and BMI are given as median (range). Incontinuous variables are given as numbers (percentage). Of the 23 patients receiving modulator therapy, 22 were treated with Lumacaftor/Ivacaftor and 1 with Tezacaftor/Ivacaftor.

Abbreviations: AX, reactance area; BMI, body mass index; deltaX5, difference between reactance at 5 and 19 Hz; FEF₂₅₋₇₅, forced expiratory flow in 25%−75% of FVC; F/F, homozygous ΔF508; F/non‐F, heterozygous ΔF508; FEV₁, forced expiratory volume in the 1st second; FOT, forced oscillations technique; FRC, functional residual capacity; FVC, forced vital capacity; LCI, lung clearance index; MBW, multiple‐breath washout; non‐F/non‐F, no ΔF508 allelle; Rrs5, resistance at 5 Hz; sGrs, specific conductance of the respiratory system; Xrs5, reactance at 5 Hz.

All FOT parameters, Rrs5, Xrs5, and AX, showed a significant improvement of approximately 1 in the z‐score 6 months following the initiation of ETI (Supporting Information S1: Table 1). The greatest improvement occurred during the first 3 months, with an additional 25% of the baseline value gained over the next 3 months. More specifically, Rrs5 z‐score decreased by 1.01 in the third month and by 1.39 in the sixth month following ETI administration (p < 0.001), Xrs5 z‐score increased by 0.72 in the third and by 0.98 in the sixth month (p < 0.001) and AX z‐score decreased by 1.08 in the third and by 1.24 in the sixth month after ETI (p < 0.001). Anthropometric parameters (height, weight, and BMI), N₂MBW, and spirometry parameters improved after ETI initiation (Supporting Information S1: Table 1). The LCI decreased by 1.48, and the FEV₁ z‐score increased by 1.64 after 6 months. Percent predicted FEV₁% (ppFEV₁) had an absolute change of +6.93%. Mean weight improved by 3.8 kg and mean BMI by 0.9 kg/m² in the sixth month of ETI treatment. sGrs and deltaX5 showed an increase of 0.192 cmH₂O^‐1·s^‐1 and 0.298 cmH₂O·s·L^‐1, respectively, but not statistically significant (p > 0.05). The changes in FEV₁ z‐score, LCI, and FOT parameters are shown in Figure 1.

Figure 1.

ANOVA plots for pulmonary tests' parameters after ETI treatment; pairwise t‐test, p. adjust: Bonferroni (*p < 0.1; **p < 0.05; ***p < 0.01). Abbreviations: LCI, lung clearance index; FEV₁, forced expiratory volume in the 1st second; Rrs5, resistance at 5 Hz; Xrs5, reactance at 5 Hz; AX, reactance area; sGrs, specific conductance of the respiratory system.

Among pwCF with different disease severity, similar trends were found between the changes in FEV₁ z‐score, Rrs5 z‐score, Xrs5 z‐score, and sGrs, based on LCI over or under 8 (Bonferroni adjustment, p > 0.05) (Figure 2). Conversely, a significant improvement of AX was noted in pwCF with LCI over 8 (p = 0.015). Based on LCI over or under 10 (Bonferroni adjustment, p > 0.05), similar trends were found between the changes in FEV₁ z‐score, Rrs5 z‐score, Xrs5 z‐score, AX z‐score, and sGrs (Figure 3).

Figure 2.

Trends in spirometric and FOT parameters changes in pwCF with LCI ≤ 8 and those with LCI > 8 (**p < 0.05; ***p < 0.01). Abbreviations: LCI, lung clearance index; FEV₁, forced expiratory volume in the 1st second; Rrs5, resistance at 5 Hz; Xrs5, reactance at 5 Hz; AX, reactance area; sGrs, specific conductance of the respiratory system. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3.

Trends in spirometric and FOT parameters changes in pwCF with LCI ≤ 10 and those with LCI > 10 (**p < 0.05; ***p < 0.01). AX, reactance area; FEV₁, forced expiratory volume in the 1st second; LCI, lung clearance index; Rrs5, resistance at 5 Hz; sGrs, specific conductance of the respiratory system; Xrs5, reactance at 5 Hz. [Color figure can be viewed at wileyonlinelibrary.com]

In multivariable panel regression analysis, the Rrs5 z‐score emerged as the most significant predictor of LCI (p < 0.05) together with the FEV1 z‐score (p < 0.05), independent of age, gender, mutation, FEF₂₅₋₇₅, Xrs5, and AX z‐score values (Table 2).

Table 2.

LCI predictors.

	Unadjusted effect			Adjusted model 1 (R 2 = 0.60)			Adjusted model 2 (R 2 = 0.61)
	β coefficient	95% CI	p value	β coefficient	95% CI	p value	β coefficient	95% CI	p value
Age	−0.006	−0.021, 0.009	0.433
Sex	0.012	−0.108, 0.132	0.841
F/F	−0.084	−0.253, 0.085	0.322
F/non‐F	−0.059	−0.154, 0.036	0.220
non‐F/non‐F	1.750	0.809, 2.691	0.000	0.077	−0.052, 0.206	0.235	0.059	−0.041, 0.159	0.241
FEV1 z‐score	0.202	−0.016, 0.420	0.068	−0.062	−0.121, −0.003	0.039	−0.077	−0.152, −0.002	0.045
FVC z‐score	0.109	−0.070, 0.288	0.227
FEF25−75 z‐score	0.047	−0.150, 0.244	0.634
Rrs5 z‐score	0.046	−0.048, 0.140	0.332	0.003	0.000, 0.006	0.044	0.027	−0.002, 0.056	0.071
Xrs5 z‐score	0.021	−0.052, 0.094	0.563
AX z‐score	0.021	−0.025, 0.067	0.367				0.011	−0.012, 0.034	0.343
R5‐19 z‐score	0.069	−0.067, 0.205	0.311	0.003	−0.003, 0.009	0.321	0.022	−0.031, 0.075	0.405
sGrs	1.380	0.039, 2.721	0.044	0.042	−0.027, 0.111	0.223	0.570	−0.139, 1.279	0.112

Abbreviations: AX, reactance area; CI, confidence interval; FEV₁, forced expiratory volume in the 1st second; F/F, homozygous ΔF508; F/non‐F, heterozygous ΔF508; FVC, forced vital capacity; FEF₂₅₋₇₅, forced expiratory flow in 25%−75% of FVC; LCI, lung clearance index; non‐F/non‐F, no ΔF508 allelle; R5‐19, difference between resistance at 5 Hz and 19 Hz; Rrs5, resistance at 5 Hz; sGrs, specific conductance of the respiratory system, Xrs5, reactance at 5 Hz.

CFTR mutation seemed to have a minor role in changes in lung function parameters after ETI. While all genotype groups demonstrated significant clinical improvement following ETI, the non‐F/non‐F genotype cohort exhibited a statistically greater magnitude of change (improvement) in LCI (p < 0.01) when statistically compared to the combined F/F and F/non‐F groups (Figure 4). In particular, the LCI improvement trend was different following 6 months of ETI between the F/F versus non‐F/non‐F and F/non‐F versus non‐F/non‐F groups (beta = −4.05, p < 0.01; beta = −3.31, p < 0.01, respectively). No other significant differences in the magnitude of change were observed among mutation groups. No signifigantly different trends were found between the changes in LCI between F/F versus F/non‐F groups (p > 0.05). No signifigantly different trends were found between the changes in FEV₁ z‐score, Rrs5 z‐score, Xrs5 z‐score, AX z‐score, and sGrs change among all mutation groups (p > 0.05). In all subjects but one, the LCI decreased following ETI initiation independently of previous modulator therapy. Twenty‐two pwCF were treated with Lumacaftor/Ivacaftor, and one patient had Tezacaftor/Ivacaftor at the initiation of ETI.

Figure 4.

Trends in spirometric and FOT parameters changes in pwCF with F/F, F/non‐F and non‐F/non‐F CFTR mutation genotypes. (**p < 0.05; ***p < 0.01). AX, reactance area; FEV₁, forced expiratory volume in the 1st second; F/F, homozygous ΔF508; F/non‐F, heterozygous ΔF508; LCI, lung clearance index; non‐F/non‐F, no ΔF508 allelle; Rrs5, resistance at 5 Hz; sGrs, specific conductance of the respiratory system; Xrs5, reactance at 5 Hz. [Color figure can be viewed at wileyonlinelibrary.com]

4. Discussion

This is the first study to explore the utility of standard lower‐frequency FOT indices in assessing the efficacy of ETI in a cohort of pwCF with varying disease severity. Our study showed that FOT, along with spirometry and N₂MBW parameters, improved significantly following ETI initiation. The most remarkable improvement of these parameters was observed in the first 3 months of treatment, while in the following 3 months, the improvement continued, albeit at a lower rate. The secondary aim of our study was to assess the trends in changes of spirometry and FOT parameters among pwCF of different disease severity and also in the three patient groups according to their mutation group, that is, F/F, F/non‐F, and non‐F/non‐F. Genotype group was found to have a minor role in ETI efficacy and in the improvement of oscillometry parameters. Disease severity based on LCI was found to have no impact on FOT and spirometry parameter improvement.

All FOT metrics improved substantially after ETI initiation, reaching an overall increase of about 1 z‐score at 6 months. The early response was most pronounced, with smaller incremental gains thereafter, indicating that ETI produces rapid initial improvements in lung mechanics that continue to evolve more gradually over time. To our knowledge, no published data exists on the change in oscillometry parameters before and after ETI therapy in pwCF. In our cohort, the absolute change in ppFEV₁ 6 months after initiation of ETI therapy was +6.93% while LCI showed a decrease of 1.4. These findings suggest that, in real‐world settings, improvements in spirometry and N₂MBW parameters among individuals with CF tend to be less pronounced than those reported in the initial trials evaluating ETI efficacy. A recent observational study conducted in pwCF eligible for ETI, aged 6−11 years, estimated real‐world changes in N₂MBW and found them different from those reported in previous clinical trials [19]. Lower baseline LCI values were recorded in the real world of pwCF, with an LCI decrease after ETI of 0.7 in comparison to previous clinical trials of Zemanick et al. [20] and Mall et al. [21] that found an LCI decrease of 1.71 and 2.23, respectively. Thus, N₂MBW parameters seem to have more modest results after ETI initiation than those reported by early clinical studies. Therefore, it is deemed necessary to redefine the change in the parameters of the respiratory tests before and after the administration of the ETI treatment. The two previously cited clinical trials [20, 21] reported increases in ppFEV₁ of 10.2% and 11%, respectively, following treatment. In contrast, a more recent study by Urquhart et al. [19] observed a more modest absolute increase of +3.1%. Our study included one participant with severe lung disease as defined by ppFEV₁ < 40% and nine with LCI > 10. Previous studies involving individuals with advanced lung disease or ppFEV₁ < 40% have demonstrated greater improvements than observed in our cohort. Burgel et al. reported a ppFEV₁ increase of +10.0 in pwCF with non‐F/non‐F genotypes [22], +15.1 in pwCF carrying at least one ΔF508 allele [23], and +17.0 in those with the N1303K mutation [24]. Notably, no individuals with the N1303K mutation were included in our study (Supporting Information S1: Table 2).

In our analysis, patterns of change in spirometry and FOT parameters were broadly comparable across disease‐severity groups, whether using an LCI threshold of 8 or 10. This suggests that ETI elicits a generally consistent physiological response across a wide spectrum of baseline ventilation inhomogeneity. However, the selective improvement observed in AX among individuals with abnormal LCI may indicate that this parameter is particularly sensitive to early changes in small‐airway function in those with greater baseline impairment. The absence of similar differential trends in the remaining FOT or spirometric indices highlights the complex and potentially heterogeneous nature of lung‐mechanical recovery following ETI. To date, the most used respiratory tests to assess the efficacy of the ETI combination are spirometry for adults and N₂MBW for paediatric patients, measuring ppFEV₁ and LCI, respectively [25]. Several studies collectively underscore the potential of LCI as a sensitive marker for early detection and monitoring of lung disease severity in CF, complementing traditional spirometry measures like FEV₁. As disease severity has improved over time, it is important to use LCI as a parameter to set disease severity [26, 27]. The selection of LCI thresholds in this study aligns with established precedents utilized in research evaluating the performance of oscillometry in pwCF [28]. The upper limit of normal LCI has been variably defined in the literature, ranging from an initial proposal of 8 [16], later revised to 7.1 [29], to a more conservative threshold of 10 utilized by subsequent studies to denote the presence of bronchiectasis [30, 31]. An increase of ≥2 units in LCI units in LCI considered indicative of pulmonary exacerbation. We employed these published cut‐off values to stratify disease severity and facilitate comparison with existing data.

We performed a multivariable panel regression analysis incorporating changes in all variables over time following ETI initiation. Our analysis identified the Rrs5 z‐score as a significant predictor of LCI (p < 0.05), second only to the FEV₁ z‐score (p < 0.01). This association was independent of age, sex, mutation type, FEF₂₅₋₇₅ z‐score, Xrs5 z‐score, and AX z‐score, with an R² value of 0.6. Fouzas et al. [28] used forced oscillometry and identified Rrs5 z‐score as a significant predictor of LCI with R [2] of 0.62 (p = 0.013). In their study, Bokov et al. [32] used impulse oscillometry and concluded that Fres is the most sensitive parameter to peripheral lung obstruction, with a sensitivity of 0.966 and a specificity of 0.636. Rrs5 was found in both studies to have the potential of predicting LCI, either among different patients at the same follow‐up visit [28] or among the same patients over time after ETI, with an ability of ≥ 60%. Recent years have shown a growing interest in comparing FOT with N₂MBW in people with CF, underscoring the expanding role of oscillometry in CF lung assessment [33]. Although evidence in adults remains limited, emerging data demonstrate good correlations between low‐frequency FOT parameters and conventional lung function measures, supporting its utility in CF care [34, 35]. In children and adolescents, earlier studies reported weaker associations between FOT, spirometry, and plethysmography [9]; however, more recent prospective study applying volume‐corrected low‐frequency FOT has shown that indices such as Xrs5 and sGrs may better detect small‐airway impairment [28]. Furthermore, FOT, in comparison with spirometry, was shown to detect respiratory dysfunction even in preschool children with CF and may also be useful in identifying more severe disease [36]. Collectively, these findings highlight the increasing relevance of FOT as a feasible, effort‐independent tool that complements N₂MBW and spirometry and may be particularly valuable in patients unable to perform spirometry.

We examined trends across mutation groups to assess whether lung function techniques differ in their sensitivity to treatment response. Mutation groups (F/F, F/non‐F, and non‐F/non‐F) reflect varying residual CFTR function that could influence both baseline physiology and response magnitude. In our cohort, despite differences in CFTR mutation status, the magnitude of lung function improvement following ETI was not statistically different between the F/F and F/non‐F groups (p > 0.05), suggesting a minor role for mutation type in modulating the therapeutic effect among these two major cohorts. The only exception was the non‐F/non‐F group, which exhibited a statistically greater magnitude of change in LCI (p < 0.01), although this finding must be interpreted cautiously due to the restricted sample size (n = 7) in this subgroup. ^{2536–383940}Modulator‐naïve patients showed significant improvements across all measured parameters, including LCI, ppFEV₁, Rrs5, Xrs5, and AX z‐scores (p < 0.01). Our findings are consistent with previous observational studies, which reported ppFEV₁ improvements of +13.2 and +15.0% in pwCF with non‐approved variants [22, 37]. Non‐F/non‐F patients showed a ppFEV₁ increase of +10.2% (+2.3; +22.5), with corresponding improvements in FOT and N₂MBW metrics. Rrs5 z‐score showed a decrease of −3.34 (−4.65; −1.82), Xrs5 z‐score an increase of +1.9 (+0.26; +4.29), AX z‐score a decrease of −1.96 (−3.16; −0.81), and LCI a decrease of −0.7 (−2.46; +0.9) in the same group. Notably, LCI improved more in modulator‐naïve non‐F/non‐F patients than in those with prior modulator use, driven in part by two individuals with marked LCI response. No other significant differences were observed among mutation groups. Our results suggest that CFTR genotype may have a limited impact on changes in pulmonary function following ETI initiation. No prior studies have assessed the utility of oscillometry in this setting. While initial ETI trials included only F/F and F/non‐F individuals [38, 39, 40], the FDA later approved use in 177 rare variants based on in vitro CFTR activation data from FRT cells [41]. At the time of our study, all seven non‐F/non‐F participants carried mutations not included in the FDA‐approved list [32] and were therefore ineligible for ETI under existing guidelines.

The small sample size, broad age range (from preschoolers to adults), and uneven distribution of patients across mutation groups and disease severities represent additional limitations that may have introduced variability into our results. Larger studies with more balanced cohorts and longer follow‐up periods are needed to confirm these findings and to better reflect the broader CF population.

In summary, the present study showed that in pwCF, changes in FOT parameters are consistent with corresponding changes in spirometry and N₂MBW independently of lung disease severity and genotype of pwCF. Spirometry and N₂MBW, to a lesser extent, require absolute cooperation from the patient, especially at preschool age, which is a challenge and may pose difficulties in adequately evaluating the effectiveness of the treatment and monitoring the course of the disease. FOT can be a helpful tool, a simple, fast, and easily reproducible method, which, with minimal cooperation, will help to successfully monitor the effectiveness of the treatment and the progression of lung disease among pwCF.

Author Contributions

Conception: Christos Kogias, Elpis Hatziagorou. Data collection: Christos Kogias, Athina Sopiadou, Petrina Vantsi, Elissavet‐Anna Chrysochoou, Evangelia Desli, Maria Galogavrou. Analysis and interpretation: Christos Kogias, Nikolaos Gasparis, Elpis Hatziagorou. Drafting the manuscript: Christos Kogias, Elpis Hatziagorou. Critical revision of the manuscript and final approval: All authors.

Funding

The authors received no specific funding for this work.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Table S1: Somatometric and pulmonary test parameters change before and 3 & 6 months after ETI treatment. Table S2: Non‐F/non‐F patients’ mutations (N=7).

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.